Object looses its scale on keyframe change
You have part of your mesh animated. When you move the frame, it displays the current transformation of the keyframe with animation.



                                  Animated Frame         Current Frame

You can fix this by going into the dope sheet, the NLA editor, or the Graph editor
And delete the unwanted animation


          You can click here to delete                      You can also select all the

          the animation for that object.                   keyframes here and delete them.

You can keep the transformation without deleting the previous keyframe by adding a new one and animating the transformation.